[w3c/clipboard-apis] Ask for tests for normative changes in CONTRIBUTING.md (#56)

See https://github.com/foolip/testing-in-standards/blob/master/policy.md
for context. This phrasing matches that used for many other specs'
CONTRIBUTING.md files.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/clipboard-apis/pull/56

-- Commit Summary --

  * Ask for tests for normative changes in CONTRIBUTING.md

-- File Changes --

    M CONTRIBUTING.md (9)

-- Patch Links --

https://github.com/w3c/clipboard-apis/pull/56.patch
https://github.com/w3c/clipboard-apis/pull/56.diff

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